The present invention relates to apparatus for converting values expressed in decimal form in which each decimal digit is expressed in binary code notation into a form in which the value is expressed entirely in binary notation.
It is well-known, for example, in connection with data processing apparatus to require such conversion. Typically data for application to such apparatus is expressed in decimal notation, and for ease of manipulation, for example for entry into the apparatus, each decimal digit is separately encoded into binary notation. However, data processing apparatus is frequently arranged so that its internal calculating transactions take place on values expressed entirely in binary code. Thus it is a requirement that the decimal values expressed by means of separately encoded digits shall be converted into a pure binary form.
It has previously been proposed to carry out this conversion by resolving each decimal digit into its binary code components and then summing the resultant components. It will be realised that the process of resolution is required to take into account the decimal denominational significance of each digit, so that, for example, the value 2 occurring in the units denomination results in a single binary component 2.sup.1 ; whereas the same value, 2, occurring in the tens denomination requires to be expressed by the binary components 2.sup.4 and 2.sup.2. The prior proposals for conversion by this resolution process have included so-called "look-up" tables which contain the various binary components required for different digits of the specified decimal denominations. However, with the development of manufacturing techniques for data storage devices, the forms of such devices have changed and it is desirable to utilize such storage devices as are commonly available in quantity for economic reasons. In particular, it is desirable to use memory elements in which the available storage capacity is expressed in a number of addressable locations each containing a predetermined number of bit storage positions. The addresses of the locations are expressed in binary notation and the bit capacity of a location is regulated by reference to a power of two. Thus, for practical purposes, it is typical for a storage element to have a bit capacity of, say, 8, and a total of 512 possible addresses. It will be appreciated, therefore, that using the binary encoded decimal digits to address such a store presents the problem that they are not compatible with the addressing system, with the result that considerable redundancy is inevitable. Moreover the capacity of a storage location is such that fewer positions are available to express binary components in any location than would be required for the expression of the theoretically greatest decimal number that the store could service.